This invention relates to RF load and source pull testing of medium and high power RF transistors and amplifiers using remotely controlled electro-mechanical impedance tuners. Modern design of high power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull”. Load pull is a measurement technique employing microwave tuners and other microwave test equipment. The microwave tuners are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested, see ref. 1; this document refers hence to “impedance tuners”, in order to make a clear distinction to “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included tuning circuits, see ref. 2.
Impedance tuners comprise, in general, a transmission line and a number of serial or parallel conductive tuning elements (probes, 22, 30, 41, 51, 52, 61) fixed or adjustable (FIGS. 2 to 5 and 6a), which, when approaching the center conductor (32) of the slabline (31) and moved along the axis of the slabline (45) create a variable reactance, allowing thus the synthesis of various impedances (or reflection factors) covering parts or the totality of the Smith chart (the normalized reflection factor area). The relation between reflection factor and impedance is given by GAMMA, (Z−Zo)/(Z+Zo), where Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is Zo=50 Ohm, see ref. 3.
Up to now such conductive or metallic probes (slugs) have been made in a cubical form (41) with a concave bottom (35) which allows capturing (when lowered towards the center conductor) the electric field which is concentrated in the area (36) between the center conductor (32) and the ground planes of the slabline (31), FIG. 3. This field capturing allows creating high and controllable reflection factors. The disadvantage of this technique is the requirement of very high precision and resolution vertical probe movement mechanisms (33, 21, 53). Because most of the field capturing effect occurs when the probe is very close to the center conductor, also a high resolution control mechanism is needed. This, on the other hand, slows down the tuning procedure, since, when the probe is away from the center conductor, the tuning effect is much less active, but the vertical moving speed is the same, see FIG. 15. In praxis, it takes typically 5 seconds to tune from GAMMA=0.05 to GAMMA=0.1 (VSWR from 1.1:1 to 1.22:1) and only 1 second to tune from GAMMA=0.9 to GAMMA=0.95 (VSWR from 19:1 to 39:1), see FIG. 15. In FIG. 15 tuning speed is proportional to the change in VSWR divided by the change in vertical position (Y): TUNING SPEED=Δ VSWR/Δ Y {eq. 1}, whereas the vertical position is directly proportional to the motor stepping speed. It can be seen (FIG. 15) that the slope of the curve (which is equivalent to the tuning speed) of the tuner increases as the probe approaches mechanical contact. When the probe is close to the center conductor, high tuning speed is unfavorable, because it corresponds to low tuning resolution (Δ VSWR per motor step). In this tuning range a rather low tuning speed (high tuning resolution) is required; whereas the opposite is true when the probe is far away from the center conductor.
The proposed new probe structure (FIGS. 6b, 7 and 8 to 12) improves on both: it simplifies the precise vertical axis and improves the tuning speed (resolution).